1. Field of the Invention
The present invention relates to a method of feeding sheets and a device for the same which are used in image forming apparatuses such as copying machines and printing machines. In particular, the invention relates to a method and a device for correcting such a state that paper sheets cannot be transported at an intended speed due to wear caused by aging of a roller for transporting the sheets and/or due to adhesion of paper powder onto a surface of the roller.
2. Description of the Related Art
In a sheet transporting system of an image forming apparatus such as a copying machine, sheets are transported by a plurality of rollers such as feed roller, transporting roller, intermediate roller, timing roller, transporting roller and discharge roller. For this sheet transporting system, there have been proposed a method and a device in which initiative in transporting the sheets is changed from a roller at an upstream position, in view of a moving direction of the sheet, to a subsequent roller pair, i.e., downstream roller pair next to the upstream roller, in accordance with a timing at which a leading edge of the sheet, which is being transported by a transporting force of the upstream roller, is pinched at a nip of the subsequent roller pair. In connection with such method and device, there has been proposed such a structure that a one-way clutch operates to idle the upstream roller after the change of initiative in transporting sheets.
In the method and device for feeding sheets described above, however, the sheet cannot be transported by the downstream roller pair and thus stops, if the upstream roller is stopped before the leading edge of the sheet, which was being transported by the transporting force of the upstream roller, is pinched at the nip of the downstream roller pair. Thus, jamming of the sheet generates. This situation, in which the upstream roller is stopped before the leading edge of the sheet is pinched at the nip of the downstream rollers, is caused, for example, by reduction of force for transporting the sheet due to the fact that slip is liable to generate because of wear caused by aging of the upstream roller and/or adhesion of paper powder.
Further, such a disadvantage may generate that the sheet slacks at a position between the upstream and downstream rollers and thereby bends, for example, into a Z-form, if the upstream roller, of which sheet transporting speed is relatively higher than that of the downstream roller, continuously applies the transporting force to the sheet even after the leading edge of the sheet is pinched at the nip of the downstream rollers and thereby the sheet starts to be transported by the transporting force of the downstream rollers. This situation, in which the upstream roller continuously applies the transporting force to the sheet even after the start of transportation of the sheet by the downstream rollers, is caused for example in the case where an excessively large margin for the transportation by the upstream roller is set in order to prevent the jamming. The situation, in which the transport speed of the downstream roller is lower than that of the upstream roller, is caused for example in such a case that slip is liable to generate on the downstream roller due to wear caused by aging and/or due to adhesion of paper powder, which results in reduction of the sheet transporting ability of the downstream roller.
Even if the transport speed of the upstream roller reduces only to an extent that does not cause the serious problem described above, the sheet being transported may not reach a predetermined position (i.e., may not be detected at the predetermined position) within a predetermined time period, in which case the apparatus determines that the jamming generates and thus stops its operation. Here, the above "predetermined time period" is a time period which is enough for the sheet, which is transported at a set speed, to reach a "predetermined position".
In view of the above, there has been proposed a method and a device for transporting sheets, in which drive, i.e., rotation of the feed roller is stopped in accordance with the timing at which a sensor opposed to a downstream roller pair detects that the leading edge of the sheet fed from the feed roller at the upstream position reaches the roller pair at the subsequent stage.
These method and device can overcome the foregoing disadvantages. However, these method and device require a complicated structure for attaching the sensor. It is difficult to attach the sensor accurately to the nip portion of the downstream roller pair without a positional error.